Several European countries widely use frequency modulation (FM) broadcasting networks for road traffic information. Additional recognition signals are superimposed on the broadcast signals which include audio signals and, in case of stereo broadcasting, stereo pilot signals, in order to indicate road traffic information service. Several different signals are used; see U.S. Pat. No. 3,949,401 Hegeler et al Apr. 6, 1976; U.S. Pat. No. 4,435,843 Eilers and Bragas Mar. 3, 1984; U.S. Pat. No. 4,450,589 Eilers and Bragas May 22, 1984; U.S. Pat. No. 319,653 filed Nov. 9, 1981 Eilers and Bragas continuation application Ser. No. 690,840, filed Jan. 14, 1985, now U.S. Pat. No. 4,584,708, issued Apr. 22, 1986.
In the FM systems, a first signal is transmitted continuously if a radio station provides traffic information service. Responding on this first signal an indication lamp or the audio channel in the receiver is switched on. Thus, when tuning a receiver a car driver can recognize whether a station will transmit traffic information. This signal is also called "program identification" signal "PI" signal for short.
A second signal also termed an announcement recognition signal, is transmitted during the traffic information service message. The second, or AR signal, indicates that a message or an announcement is being transmitted, in contrast to other program contents. This signal is also called "message signal", "ME" signal for short. By means of this AR signal the volume of the receiver can be increased or the receiver can be switched over from tape reproduction to radio reception.
A third signal is transmitted continuously in order to identify the station, area, or the region in which the station provides traffic information service. Therefore this signal is called radio station or region recognition signal or RR signals.
The transmission of these additional signals has to be compatible with the established mono and stereo broadcasting systems. That means the additional signals must not interfere with the audio signals and the pilot one in case of stereo and vice versa. Furthermore the additional signals must not exceed the frequency band and amplitude limits allowed for broadcast transmitters by the applicable rules, e.g. in the USA the FCC rules. Thus, the modulation indices must stay within predetermined permitted limits.